Microbiologically influenced corrosion (MIC) of stainless steel 304 by a marine aerobic Pseudomonas bacterium in a seawater-based medium was investigated by atomic force microscopy (AFM) and X-ray photoelectron spectroscopy (XPS). AFM was used to observe in situ the proliferation of a sessile Pseudomonas cell by binary fission. The development of a biofilm on the coupon surface and the extent of corrosion damage beneath the biofilm after various exposure times were also characterized by AFM. Results showed that the biofilm formed on the coupon surface increased in thickness and heterogeneity with time, and thus resulting in the occurrence of extensive micro-pitting corrosion; whilst the depth of pits increased linearly with time. The XPS results confirmed that the colonization of Pseudomonas bacteria on the coupon surface induced subtle changes in the alloy elemental composition in the outermost layer of surface films. The most significant feature resulting from microbial colonization on the coupon surface was the depletion of iron (Fe) and the enrichment of chromium (Cr) content as compared to a control coupon exposed to the sterile medium, and the enrichment of Cr increased with time. These compositional changes in the main alloying elements may be correlated with the occurrence of extensive micropitting corrosion on the surface.